Sexually dimorphic expression of vasa isoforms in the tongue sole (Cynoglossus semilaevis)

Vasa Is a Potential Germ Cell Marker in Leopard Coral Grouper (Plectropomus leopardus)

Vasa (Ddx4, DEAD box polypeptide 4), an extremely specific marker of germ cells in vivo, is an ATP-dependent RNA helicase that plays an essential role in germ cell development and gametogenesis. However, the expression and function information about this gene in groupers remains lacking. Here, vasa homolog termed Plvasa was isolated and identified Plvasa as a putative germ cell marker in the leopard coral grouper, (Plectropomus leopardus). Results indicated that Plvasa contained 17 exons in the genomic sequence and 9 conserved motifs of the DEAD-box protein by sequence analysis. The sequence comparison, phylogenetic analyses and synteny analyses showed that Plvasa was homologous with other teleosts. Additionally, the expression of Plvasa was significantly higher in gonads than in other tissues in adult individuals (p < 0.05). Further, the distribution of Plvasa revealed that it was only expressed in the germ cells, such as spermatids, germline stem cells and oocytes at different stages, and could not be detected in the somatic cells of gonads. The current study verified that the Plvasa gene is a valuable molecular marker of germ cells in leopard coral grouper, which potentially plays an important role in investigating the genesis and development of teleost germ cells.

Cloning and Expression Profiling of the Gene vasa during First Annual Gonadal Development of Cobia (Rachycentron canadum)

The vasa gene is essential for germ cell development and gametogenesis both in vertebrates and in invertebrates. In the present study, vasa (Rcvasa) cDNA was cloned from cobia (Rachycentron canadum) using the RACE amplification method. We found that the full-length cDNA sequence of Rcvasa comprises 2571 bp, containing a 5′-UTR of 145 bp, a 3′-UTR of 341 bp, and an open reading frame (ORF) of 2085 bp, encoding a protein of 694 aa. The deduced amino acid sequence contains 8 conserved motifs of the DEAD-box protein family, 7 RGG repeats, and 10 RG repeats in the N-terminal region. Comparisons of the deduced amino acid sequence with those of other teleosts revealed the highest percentage identity (86.0%) with Seriola quinqueradiata. By using semiquantitative RT-PCR, Rcvasa appeared to be specifically expressed in the testis and ovary, among 13 tissues analyzed. In addition, annual changes in Rcvasa expression levels were examined in the gonads by quantitative real-time PCR (qRT-PCR). The expression of Rcvasa in the testis first increased significantly at 120 dph (stage II–III), then stabilized as the testis developed from 185 dph (stage III) to 360 dph (stage V). During the development of the ovary (stage I to II), the expression of Rcvasa first increased and reached the highest level at 210 dph (stage II), then decreased. Furthermore, the results of chromogenic in situ hybridization (CISH) revealed that Rcvasa mRNA was mainly expressed in germ cells and barely detected in somatic cells. In the testis, Rcvasa mRNA signal was concentrated in the periphery of spermatogonia, primary spermatocytes, and secondary spermatocytes and was significantly weaker in spermatids and spermatozoa. In the ovary, Rcvasa mRNA signal was uniformly distributed in the perinuclear cytoplasm and was intense in early primary oocytes (stage I and II). These findings could provide a reference for understanding the regulatory mechanisms of vasa expression during the development of germ cells in cobia.

Characterization of kiss2 and kissr2 genes and the regulation of kisspeptin on the HPG axis in Cynoglossus semilaevis

Reproduction allows organisms to produce offspring. Animals shift from immature juveniles into mature adults and become capable of sexual reproduction during puberty, which culminates in the first spermiation and sperm hydration or ovulation. Reproduction is closely related to the precise control of the hypothalamic-pituitary-gonadal (HPG) axis. Kisspeptin peptides are considered as the important regulator of HPG axis in mammalian. However, the current understanding of kisspeptin in flatfish is not comprehensive. In this study, we cloned and analyzed the kiss2 and kissr2 genes in Cynoglossus semilaevis. Interesting alternative splicing in the 5'-untranslated regions (UTR) of the Cskissr2 gene was found. The expression profiles of Cskiss2 and Cskissr2 showed relative high messenger RNA (mRNA) levels at the late gastrula stage during embryonic development, at total length = 40 mm during early gonadal differentiation, and in the brains and gonads of all investigated tissues. These results suggested that the kisspeptin system participated in embryogenesis and in the regulation of gonadal differentiation and development. Considering that the control and regulatory mechanisms of kisspeptin in the central reproductive axis are still unclear, we documented that the intramuscular injection of kisspeptin caused different sGnRH and cGnRH mRNA levels in a dose- and tissue-dependent manner. The mRNA expressions of FSH and LH were stimulated in the ovary and were inhibited in the testis under the kisspeptin treatments. These results provided foundations for understanding the roles of kisspeptin in the neuroendocrine system in fish. The manipulation of the kisspeptin system may provide new opportunities to control the gonadal development and even reproduction in fish.

Transcriptome analysis of the gonads of olive flounder (Paralichthys olivaceus)

Olive flounder (Paralichthys olivaceus) is an economically important cultured marine fish in China, Korea, and Japan. Controlling and managing the breeding of olive flounder in captivity is an imperative step toward obtaining a sustainable supply of this fish in aquaculture production systems. Therefore, investigation on the molecular regulatory mechanism of gonadal development and gametogenesis in this species is of great significance in aquaculture. Furthermore, identification of the expression profile of numerous sex-related genes is the first step to primarily understand such molecular regulatory mechanism. Six female and six male gonads obtained from 2-year-old olive flounders were sequenced using Illumina, which produced 6.68 and 6.24 GB data for ovary and testis, respectively. The reads were mapped to the draft genome of olive flounder, and then the reads per kilobase per million (FPKM) for each gene were calculated. The female-/male-biased expressed genes were investigated based on the FPKM values. Overall, 3086 female-biased and 5048 male-biased genes were screened out. GO enrichment analysis showed that the GO terms "male meiosis," "gamete generation," "fertilization," "spermatogenesis," and "germ plasma" were enriched in male-biased genes. In addition, the GO terms "cell morphogenesis involved in differentiation," "embryonic morphogenesis," "plasma membrane," "steroid hormone receptor activity," and "aromatase activity" were enriched in female-biased genes. Moreover, 373,369 single nucleotide polymorphisms and 32,993 indels were identified in the transcriptome. This work is the largest collection of gonad transcriptome data for olive flounder and provides an extensive resource for future gonadal development and gametogenesis molecular biology studies in this species.